1. Field of the Invention
This invention relates in general to nuclear reactor control systems and, in particular, to systems for controlling the movement of nuclear control rods into and out of the core of a nuclear reactor.
2. Description of the Prior Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred into a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity with one another in a fuel assembly structure through and over which the coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge on the fissile material in an adjacent fuel rod, contributing to the nuclear reaction and to the heat generated by the core.
Moveable control rods are dispersed throughout the nuclear core to enable control of the overall rate of fission, by absorbing a portion of the neutrons passing between fuel rods, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to fission in an adjacent fuel rod; and retracting the control rod reduces the extent of neutron absorption and increases the rate of the nuclear reaction and the power output of the core.
The control rods are supported in cluster assemblies that are moveable to advance or retract a group of control rods relative to the core. For this purpose, control rod drive mechanisms are provided, typically as part of an upper internals arrangement located within the nuclear reactor vessel above the nuclear core. The reactor vessel is typically pressurized to a high internal pressure, and the control rod drive mechanisms are housed in pressure housings that are tubular extensions of the reactor pressure vessel. FIG. 1 is a schematic view of a prior art nuclear containment 10 housing a reactor pressure vessel 12 having a nuclear core 14 supported within the lower half of the pressure vessel 12. A control rod assembly 16 is shown within the core 14 and supports a cluster of control rods 18 that are moved into and out of the fuel assemblies (not shown) by a drive rod 20. The drive rod 20 is moveably supported by a drive rod housing 24 that extends upwardly and through a removable reactor closure head 22. Control rod drive mechanisms (CRDM) are positioned above the reactor head around the control rod drive rod housing 24 and moves the drive rods in a vertical direction to either insert or withdraw the control rods 18 from the fuel assemblies within the core 14. Rod position indicator coils 26 or other indicator mechanisms are positioned around the housing 24 to track the position of the drive rod 20, and thus the control rods 18 relative to the core 14. The output of the rod position indicator coils 26 is fed through a processor rod position indicator (RPI) electronics cabinet 28 within the containment 10. The output of the rod position indicator electronics cabinet 28 is then fed outside the containment to a logic cabinet 30 and an RPI processing unit 32. The logic cabinet 30 interfaces with the reactor control system 34 which provides manual instructions from a user interface 36 as well as automatic instructions which it generates from the intelligence from plant sensors not shown. The logic cabinet 30 receives manual demand signals from an operator through a user interface 36 and reactor control system 34 or automatic demand signals from the reactor control system 34 and provides the command signals needed to operate the control rods 18 according to a predetermined schedule. The power cabinet 38 provides a programmed current to operate the CRDM, all in a well-known manner.
One type of mechanism for positioning a control rod assembly 16 is a magnetic jack-type mechanism, operable to move the control rod drive rod by an incremental distance into or out of the core in discrete steps. In one embodiment, the control rod drive mechanism has three electromagnetic coils and armatures or plungers that are operated in a coordinated manner to raise and lower a drive rod shaft 20 and a control rod cluster assembly 16 coupled to the shaft 20. The three coils (CRDM) are mounted around and outside the pressure housing 24. Two of the three coils operate grippers that when powered by the coils engage the drive rod shaft, with one or the grippers being axially stationary and the other axially moveable.
The drive rod shaft has axially spaced circumferential grooves that are clasped by latches on the grippers, spaced circumferentially around the drive rod shaft. The third coil actuates a lift plunger coupled between the moveable gripper and a fixed point. If power to the control rod mechanism is lost, the two grippers both release and the control rods drop by gravity into their maximum nuclear flux damping position. So long as control power remains activated, at least one of the stationary gripper and the moveable gripper holds the drive rod shaft at all times.
The three coils are operated in a timed and coordinated manner alternately to hold and to move the drive shaft. The sequence of gripping actions and movement is different depending on whether the step-wise movement is a retraction or an advance. The stationary gripper and the moveable gripper operate substantially, alternately, although during the sequence of movements both grippers engage the drive shaft during a change from holding stationary to movement for advance or retraction. The stationary gripper can hold the drive shaft while the moveable gripper is moved to a new position of engagement, for lowering (advancing) the drive shaft and the control rods. The moveable grippers engage the drive shaft when moving it up or down as controlled by the lift plunger. After the moveable gripper engages the drive shaft, the stationary gripper is released and then the plunger is activated or deactivated to effect movement in one direction or the other. Typically, each jacking or stepping movement moves the drive rod shaft ⅝ inch (1.6 cm), and some 228 steps are taken at about 0.8 seconds per step, to move a control rod cluster over its full span of positions between the bottom and the top of the fuel assembly.
More particularly, for lifting (retracting) the control rods, the following steps are accomplished in sequence, beginning with the stationary gripper engaged in a drive rod groove and the moveable gripper and plunger both being deactivated:                1. The moveable gripper is energized and engages the drive rod groove;        2. The stationary gripper is de-energized and disengages from the drive rod;        3. The lift coil is energized and electromagnetically lifts the moveable gripper and the drive rod an elevation equal to the span of the lift plunger;        4. The stationary gripper is energized, re-engages and holds the drive rod (i.e., both grippers are engaged);        5. The moveable gripper is de-energized and disengages the drive rod; and        6. The lift coil is de-energized, dropping the moveable gripper back to its start position, mainly one step lower on the lifted drive rod.        
Similarly, for lowering (advancing) the control rods, the following steps are accomplished in sequence, again beginning with only the stationary gripper energized:                1. The lift coil is energized, moving the moveable gripper one step up along the drive rod;        2. The moveable gripper coil is energized and the moveable gripper grips the drive rod;        3. The stationary coil is de-energized, releasing the drive rod;        4. The lift coil is de-energized, dropping the moveable gripper and the drive rod by one step;        5. The stationary coil is energized and the stationary gripper engages the drive rod, at a position one step higher than its previous position; and        6. The moveable coil is de-energized and the moveable gripper disengages from the drive rod.        
A number of particular coil mechanisms and gripper mechanisms are possible. Examples of coil jacking mechanisms with a stationary gripper, a moveable gripper and a lifting coil as described heretofore are disclosed, for example, in U.S. Pat. Nos. 5,307,384, 5,066,451, and 5,009,834. In addition, four and five-coil linear drive mechanisms have been employed that operate in a similar manner, such as that described in U.S. Pat. No. 3,959,071.
Whatever mechanical arrangement is employed for the grippers and lifting coil/armature arrangement, a discrete time interval is needed to complete each sequential operation. In order to move the control rods quickly, reliably and efficiently, the respective grippers and coils must be operated accurately as to their timing. This requires that the coil energizing electric power signals to the respective coils be accurately timed.
The power level of coil energization can be simply on and off, or preferably, the coils can be energized at different levels during different operations in the sequence. The coil signals are switched between the levels in a coordinated manner by a logic controller. The logic controller generates timing signals to switch power regulation circuits on and off or between current levels as more fully described in U.S. Pat. No. 5,999,583.
The current rod control system designs for many nuclear power plants were developed during the 1970s. These systems have many single failure mechanisms any one of which can lead to dropped rods. The systems were designed prior to, and do not take advantage of, the capabilities of modern computer-based instrumentation and control equipment. Furthermore, these systems are costly to manufacture and maintain and an improved rod control system that overcomes these and other limitations is needed. Preferably, such a rod control system will be flexible enough to adapt to different magnetic jack system designs without significant re-engineering. Furthermore, such a system should be capable of being retrofitted to existing magnetic jack mechanisms.